G10711 High Precision Os on Phoenix X62 TIMS

G10711 High Precision Os on Phoenix X62 TIMS

High precision Os isotope ratio measurements using Phoenix X62 TIMS. Application Brief G10711 Zenon Palacz & Colin Fenwick, Isotopx Ltd, Middlewich, Cheshire, UK. 1. Os Isotopes Introduction Two decay schemes are important in Os isotope analysis. The study of Osmium isotopes is an important tool in 1. 187Re decays to 187Os with a half life ~42 Gyr. geochemistry and cosmochemistry (e.g. Shirey and Walker 2. 190Pt decays to 186Os with a half life ~ 470 Gyr. 1998, Brandon et al. 2006, Ravizza and Turekian 1992). Side box 1 provides background on the two Os isotope 187Os and 186Os are quoted as a ratio against stable 188Os systems of interest. Re/Os – Re is enriched in the crust relative to the mantle leading Whilst 187Os/188Os ratios can vary by several percent, the to wide variations in 187Os/186Os of surface rocks. Low Os variation in 186Os/188Os is far smaller – a few hundred ppm. concentrations and complex chemistry prior to NTIMS analysis Consequently any meaningful 186Os/188Os measurements remain major challenges. must be capable of attaining a precision of ~ 20ppm Pt/Os – The extremely long half life of 190Pt and a comparatively (2SRSD). For Faradays this implies a 186Os beam intensity narrow range in Pt/Os means that even small variations in of 100mv or more. 186Os/188Os are significant. Very high measurement precision is Phoenix TIMS therefore required. We present here the first high precision Osmium isotope ratio data from the Phoenix Thermal Ionization Mass 2. NTIMS Analysis Spectrometer. Osmium is measured by TIMS as a the negatively ionized OsO3¯ Side box 2 provides details of the NTIMS methodology molecular species. This is because the ionization potential of used. The aim of the study was to provide a performance Osmium is so high that it would evaporate before Os+ ions could baseline using a standard Phoenix instrument situated in a form (Creaser et. al 1991). Osmium has a high electron affinity, - factory environment. There was no temperature or and will readily combine with O . The formation of OsO3¯ requires environmental control. a high work function filament (Platinum), and a low work function activator Ba(NO3¯)2 or Ba(OH)2 or a combination. The activator New Developments covers the Osmium on the filament and during thermal ionization it Phoenix includes three specific developments which make - disassociates to combine the O with Os. Formation of OsO3¯ is these measurements possible:- also facilitated by bleeding a very small quantity of Oxygen close 1. A newly designed focus unit which can analyse to the sample filament. Addition of oxygen from the bleed system negative ions at up to -8KV. -6KV was used throughout raises the source vacuum from 1e-8mbar to 2e-7mbar. The small this work as it did not reduce the sensitivity. magnet placed next to the Pt filament deflects electrons away 2. Small Al-Ni-Co magnets have been fitted to one of the from the mass spectrometer lens stack, thereby ensuring focus side filament (Figure 1). This facilitates the repulsion of continuity over a range of filament currents. electrons as close as possible to the filament. Large electron beams can produce irregularities in the peak Peak Shape Stability 192 shape and potentially a high voltage flashover. Figure 2 shows a magnet scan over OsO3¯ (the major Os oxide 3. A targeted oxygen bleed system which directs oxygen isotope peak at mass 240) repeated continuously for 45 minutes. immediately adjacent to the sample bead holder. The quality and stability of the peak shape highlights the performance of the new focusing electronics. The peak side stability was better than 10ppm of mass over the 45 minutes. 192 Figure 1. The New side filament Figure 2. OsO3¯ peak shape recorded continuously for 45 mounted magnet. minutes on the axial Faraday. www.isotopx.com Application Brief G10711 ¯ Multi-dynamic remains the gold standard mode as it eliminates OsO3 Coincidence any variations in collector efficiency or amplifier gain drift. The Figure 3 shows a magnet scan from m/z 233.5 to m/z 241.5 multidynamic method requires that normalizing isotopes are (referenced to axial collector) showing the response of all 9 measured on the same collectors as the isotope ratios of Faradays on one diagram. For this scan all 9 Faradays have interest. This requires the normalizing isotopes to be closely been set to the equivalent of 1 AMU spacing for OsO3¯ matched in mass to the isotopes of interest. For example a true At the extreme left hand side of the figure the traces for the 5 multidynamic analysis which cancels all collector gains and 187 186 channels from Axial to H4 show coincidence of peaks from efficiencies could be made for 235/236 ( Os/ Os) in cycle 1 186 ¯ and 2 (highlighted on Table 1) using a mass fractionation m/z 234 (predominantly OsO3 ) up to m/z 238 190 ¯ correction from 238/236 (190Os/188Os) in cycle 3. (predominantly OsO3 ). At this point the L5 Faraday is 182 ¯ collecting m/z 230 ( WO3 ) although none is evident. However, the current consensus is that mass fractionation On the extreme right of Figure 3, mass 241 is on the Axial should use the stable isotope ratio pair with the largest mass 189 difference, i.e. 240/236 (190OsO ¯/186OsO ¯) as this produces collector, mass 245 is on H4 and mass 237 ( OsO3¯) is on 3 3 more precise normalised osmium isotope ratios (Brandon et. al. the L5 Faraday. Hence with its 9 Faradays set to OsO3¯ spacing, Phoenix can achieve coincidence on isotopes from 2006). It is evident from Table 1 that (for example) the 235/236 m/z 230 to m/z 245. does not have a multi-dynamic solution using mass fractionation correcting with 240/236. The multi-static approach is relatively simple. Static ratios are measured in each collector pair, an average is calculated and a fractionation correction applied using the average 240/236 ratio from each of the 4 sequences. The method provides partial cancellation of gain and collector efficiency variations by measuring the same isotope ratios on different collector pairs. A rigorous comparison with multi- dynamic analysis is under investigation as the methodology will have application to other isotope systems such as Nd and Sr. Sample loading Method Two samples were used. An Osmium Standard kindly provided by Dr. Richard Walker from the University of Maryland, and an ICP Osmium standard from Alfa chemicals. Both samples were in an HCl matrix. Sample sizes for the UMd 234 235 236 237 238 239 240 241 standard were typically 140ng, while the Alfa standard was 500ng. Figure 3. Scan of OsO3¯ across all 9 Faradays. Mass number relates to the Axial Faraday. Samples were loaded onto Platinum ribbon and dried down with no filament current. 2 microlitres of Barium hydroxide was placed onto the Osmium and dried down at 1A. Multi-static analysis Mass Spectrometry We have adopted a 4 cycle multi-static analysis routine as The filament current was increased steadily to 2 Amps over the shown in Table 1. Multi-static is a new acquisition method 35 currently under investigation at Isotopx. course of 2 hours. The Cl isotope was monitored until it had grown to ~6 volts. At this stage the Osmium ion beam started to appear. The filament current was then raised slowly over the Cycle course of 1 hour to ~2.4A, by this time the Osmium ion signal 1 2 3 4 had grown to >3volts. H4 240 Data collection started at ~720C and continued for 150 cycles. Each cycle consisted of 4 mass jumps with an integration of 15 H3 239 240 seconds for each mass. Baselines were taken at the start of each block. These were measured for 30 seconds at 0.5amu H2 238 239 240 either side of the peak. The ion beam was focussed and peak H1 237 238 239 240 centred at the start of each block of data. Generally the ion beam remained steady during the course of the measurement Ax 236 237 238 239 with no requirement to increase the filament current. L2 235 236 237 238 During the measurement, oxygen was bled into the source of -7 L3 234 235 236 237 the Phoenix to provide a source vacuum of 2e mbar. The base source vacuum was <1e-8 mbar, no cryo-cooling of the source L4 233 234 235 236 was required. Analyser vacuum remained at <5e-9 mbar during L5 232 233 234 235 this process. Each analysis took ~ 6 hours, with data collection taking 50% of the time. Table 1. Multi-static analysis protocol. www.isotopx.com Application Brief G10711 Following correction for 187Re on 187Os, all the isotopes were 3 Oxygen isotope corrections corrected for oxygen isotopes. Side box 3 provides details on Because the isotopes of osmium are measured as a tri-oxide the O2 correction used. The corrected ratios were then species, accurate correction for 17O and 18O has to be made. fractionation corrected using an exponential law and a 192 188 These corrections have been described extensively elsewhere Os/ Os ratio of 3.083,( Brandon et al. 2006). e.g. (Luguet et. al 2008). For this study a fixed oxygen ratio was Results used which had been determined from analysis of 242/240 and The results are shown in Table 3 and in Figures 4 and 5. 241/240 on a separate series of analyses. It was found that the ratio was extremely stable over a period of a week and so a fixed 186 188 Os/ Os – Results for both samples show excellent value was used for the measurements presented here.

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